1. Field of the Invention
The present invention relates to digital radio link receiving device.
The term "digital radio link" is generally used to designate the channel used for transmitting digital information. This radio link is hence made up by a send device, a receive device, and the propagation space separating them.
2. Brief Description of Prior Art
A conventional digital receiver is normally composed of three parts:
a first microwave frequency part including a low noise amplifier adapted to reduce the noise factor of the whole system by the maximum amount possible, a mixer for downconverting the microwave signal to an intermediate frequency (typically 70 or 140 Megahertz), and microwave frequency conversion or transposition oscillator;
an intermediate part including a low noise amplifier, a channel filter, a group propagation delay corrector and a transposition oscillator, a digital demodulator and a demodulation oscillator; and
a baseband part comprising in particular a device for carrier recovery at intermediate frequency.
This type of receiver has the advantage of possessing a wide passband which is only limited by the passband of the microwave frequency low noise amplifier. On the other hand, however, it suffers from the drawback of including a large number of functional parts which penalizes it both as regards its power consumption and cost, as well as the volume it occupies. Moreover, the filter employed at intermediate frequency must possess arithmetic symmetry both in transmission as well as in group propagation delay, and is consequently very difficult to make bearing in mind the fact that for such frequencies, only the construction of passband filters having geometrical symmetry is something that is readily accessible.
Additionally, in accordance with French patent application FR-A-2 543 379, filed Mar. 13, 1983 digital receivers exist which, as they employ direct demodulation at microwave frequency, do not use downconversion to an intermediate frequency. A receiver of this type consists of two parts:
a microwave frequency part including a low noise amplifier, a microwave frequency demodulator and a demodulation microwave frequency oscillator; and
a baseband part which includes, in particular, a phase estimator as well as for each demodulated channel, a low noise amplifier, a filter, a group propagation delay corrector, and an Automatic Gain Control amplifier.
This type of receiver suffers from the disadvantage of having such a small passband that this receiver can only practically be used in one single transmission channel. This passband is actually limited by the passband of the microwave frequency demodulator, which is small as it is not know how to provide an accurate microwave frequency phase shifter that includes a wide passband. Additionally, this microwave frequency modulator has a complex design and is difficult to set up and adjust.